diff -ru a/src/runtime/hashmap_fast.go b/src/runtime/hashmap_fast.go
--- a/src/runtime/hashmap_fast.go	2018-04-30 13:26:49.000000000 -0700
+++ b/src/runtime/hashmap_fast.go	2018-07-30 15:40:09.478159224 -0700
@@ -361,7 +361,7 @@
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapassign.
-	h.flags |= hashWriting
+	h.flags = h.flags ^ hashWriting
 
 	if h.buckets == nil {
 		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -447,7 +447,7 @@
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapassign.
-	h.flags |= hashWriting
+	h.flags = h.flags ^ hashWriting
 
 	if h.buckets == nil {
 		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -533,7 +533,7 @@
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapassign.
-	h.flags |= hashWriting
+	h.flags = h.flags ^ hashWriting
 
 	if h.buckets == nil {
 		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -619,7 +619,7 @@
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapassign.
-	h.flags |= hashWriting
+	h.flags = h.flags ^ hashWriting
 
 	if h.buckets == nil {
 		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -706,7 +706,7 @@
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&s)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapassign.
-	h.flags |= hashWriting
+	h.flags = h.flags ^ hashWriting
 
 	if h.buckets == nil {
 		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -797,7 +797,7 @@
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapdelete
-	h.flags |= hashWriting
+	h.flags = h.flags ^ hashWriting
 
 	bucket := hash & bucketMask(h.B)
 	if h.growing() {
@@ -846,7 +846,7 @@
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapdelete
-	h.flags |= hashWriting
+	h.flags = h.flags ^ hashWriting
 
 	bucket := hash & bucketMask(h.B)
 	if h.growing() {
@@ -896,7 +896,7 @@
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&ky)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapdelete
-	h.flags |= hashWriting
+	h.flags = h.flags ^ hashWriting
 
 	bucket := hash & bucketMask(h.B)
 	if h.growing() {
diff -ru a/src/runtime/hashmap.go b/src/runtime/hashmap.go
--- a/src/runtime/hashmap.go	2018-04-30 13:26:49.000000000 -0700
+++ b/src/runtime/hashmap.go	2018-07-30 13:14:06.137691211 -0700
@@ -523,7 +523,7 @@
 
 	// Set hashWriting after calling alg.hash, since alg.hash may panic,
 	// in which case we have not actually done a write.
-	h.flags |= hashWriting
+	h.flags = h.flags ^ hashWriting
 
 	if h.buckets == nil {
 		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -635,7 +635,7 @@
 
 	// Set hashWriting after calling alg.hash, since alg.hash may panic,
 	// in which case we have not actually done a write (delete).
-	h.flags |= hashWriting
+	h.flags = h.flags ^ hashWriting
 
 	bucket := hash & bucketMask(h.B)
 	if h.growing() {